An average of 132 millimetres of rain has fallen in the Manning River in the past two days to 11pm on Saturday.
Another 10 to 30 millimetres is forecast for the next six to nine hours.
Minor flooding is expected at Gloucester tomorrow morning. Moderate flooding is expected at Wingham and Taree.
At this stage it is not possible to predict the flood peak because of uncertainty over how much more rain will fall.
It is expected the Gloucester River at Gloucester will reach 4.3 metres early Sunday morning with minor flooding.
The Manning River at Wingham is predicted to reach 9.3 metres around 2am Sunday with moderate flooding.
And in Taree the Manning River is expected to reach 3.1 metres around 5am Sunday with moderate flooding.
FloodSafe advice is available at www.ses.nsw.gov.au and for emergency assistance call the SES on telephone number 132 500.
For life-threatening emergencies, call 000 immediately.